подмышка

под- (pod-, “under”) +‎ мы́шка (mýška, “"mouse", meaning "muscle" (see мы́шца (mýšca) for more)”)

• IPA^(key): [pɐdˈmɨʂkə]

подмы́шка • (podmýška) f inan (genitive подмы́шки, nominative plural подмы́шки, genitive plural подмы́шек)

1. (anatomy) armpit, underarm

   Synonym: подмышечная я́мка (podmyšečnaja jámka)

   Я хочу́ ню́хать его́ волоса́тые подмы́шки.

   Ja xočú njúxatʹ jevó volosátyje podmýški.

   I want to sniff his hairy armpits.

   Я прячу книгу под мышкой.

   Ja prjaču knigu pod myškoj.

   I hide a book in my armpit.

• When preceded by the prefix под (pod, “under”), the first part of the word is omitted:

  Я несу портфель под мышкой. ― Ja nesu portfelʹ pod myškoj. ― I am carrying a briefcase in my armpit.

  Я положу этот мешок себе под мышку ― Ja položu etot mešok sebe pod myšku ― I will put this bag in my armpit.

This seems to be a historically, rather than grammatically, defined feature, since originally the main form was simply мышка. This historical noun is still present in other Slavic languages, cf. Macedonian мишка (miška).